Many wireless communication systems have adopted orthogonal frequency-division multiplexing (OFDM) because of its ability to effectively convert frequency selective channel to multiple flat subchannels by one tap frequency domain channel equalization, adaptive and flexible bandwidth utilization and higher spectral efficiency with overlapping subcarriers.
Although robustness against frequency selectivity gave it advantage in multipath channels, OFDM systems may be subjected to frequency and/or time dispersive channel conditions. Rectangular pulse shape adopted by OFDM corresponds to sinc-shaped subcarriers, which have high sidelobes along frequency with zero crossing property at each subcarrier spacing. Overlapping sinc-shaped subcarriers make OFDM very sensitive to inter-carrier interference (ICI) in the presence of frequency domain impairments like Doppler spread and carrier frequency offset (CFO). Thus, ICI is an example of a frequency-based interference that is caused by frequency dispersion.
OFDM systems may also experience time-based interference. An example of time-based interference is inter-symbol interference (ISI). Inserting a guard interval, cyclic prefix (CP) that is copied from the end of a symbol with a length of greater than the maximum excess delay of the multipath channel, to the beginning of each symbol provides circular convolution of signal with channel, which makes frequency domain equalization without ISI possible. Any CP period shorter than the multipath channel length leads to both ISI from previous symbol and ICI due to loss of orthogonality between subcarriers.
Interference awareness and identification are problems in orthogonal frequency-division multiplexing based systems, with increased number of users along with excessive capacity demand.